1. Field of the Invention
The present invention relates generally to fabrication of semiconductor integrated circuits and, in particular, to use of germanium in substitutional sites in a crystalline silicon lattice as a diffusion barrier for both n-type and p-type dopant species.
2. Discussion of the Prior Art
The technology of semiconductor integrated circuits is based upon controlling electric charge in the surface region of the semiconductor material used as the starting substrate for the circuits. In the vast majority of integrated circuits manufactured today, the semiconductor substrate is pure crystalline silicon.
Since a silicon crystal structure consists of a regular pattern of silicon atoms, its atomic arrangement can be described by specifying silicon atom positions in a repeating unit of the silicon lattice.
FIG. 1 shows the cubic unit "cell" of crystalline silicon, identifying eight corner silicon atoms (C), eight exterior face silicon atoms (F) and four interior silicon atoms (I). It can be seen from FIG. 1 that if a given silicon atom is considered as being at the center of a box, then that silicon atom is bonded to four neighboring silicon atoms that define the four vertices of an equilateral pyramid. Five silicon atoms in the upper front corner of the FIG. 1 unit cell are accentuated to illustrate the basic building block. This building block may then be used to construct the cubic cell shown in FIG. 1, the so-called diamond cubic crystal structure.
The silicon diamond cubic crystal structure shown in FIG. 1 has the property that it may be repeated in three mutually perpendicular directions to generate a silicon crystal of the desired size.
It also has the property that it may be represented by closely-packed planes of atomic spheres stacked one on top of another to maximize density. A packing arrangement of this type results in the definition of both tetrahedral combinations of spheres and octahedral combinations of spheres. A closely-packed, repetitive array of tetrahedra contains octahedral interstitial spaces between the tetrahedra. Similarly, a closely-packed, repetitive array of octahedral contains tetrahedral interstitial spaces. Thus, a crystalline silicon lattice is said to include both tetrahedral and octahedral interstitial sites.
Control of electric charge in the surface region of crystalline silicon used for fabricating integrated circuits is achieved by introducing impurity or "dopant" atoms into the silicon lattice. Depending on the desired electrical characteristics, the dopant may be either "n-type" or "p-type."
When a dopant is introduced that has five valence electrons, i.e., one more valence electron than silicon, an extra electron is provided that does not fit into the bonding scheme of the silicon lattice. This extra electron can be used to conduct current. The "n-type" dopant atoms (e.g. phosphorous (P), arsenic (As) and antimony (Sb)), are called donors because, as Group V elements, they possess this extra electron. The "n" denotes negative and is used to represent the surplus of negative charge carriers available in the silicon lattice with the dopant present.
When a dopant is introduced that has only three valance electrons, a place exists in the silicon lattice for a fourth electron. This "p-type" dopant (such as boron) is called an acceptor. The "p" denotes positive and represents the surplus of "holes", or positive charge carriers, that exists in the lattice.
In the fabrication of integrated circuits, dopants are often introduced to the silicon lattice by diffusion. Diffusion is the mechanism by which different sets of particles confined to the same volume tend to spread out and redistribute themselves evenly throughout the confined volume. In the case of integrated circuits, the diffusion process results in movement and distribution of dopant atoms in the crystalline silicon lattice. In crystalline solids, diffusion is significant only at elevated temperatures where the thermal energy of the individual lattice atoms becomes great enough to overcome the interatomic forces that hold the lattice together.
In crystalline silicon, dopants diffuse through the lattice by one of two diffusion mechanisms substitutional diffusion or interstitial diffusion, or by a combination of the two. By substitutional diffusion, the dopant atoms move through the lattice by replacing a silicon atom at a given lattice site. By interstitial diffusion, the dopant atoms move via the tetrahedral or octahedral interstitial sites in the lattice structure.
According to Fick's first law, particles tend to diffuse from a region of high concentration to a region of lower concentration at a rate proportional to the concentration gradient between the two regions. This can be mathematically expressed as: ##EQU1## where F is the net particle flux density, N is the number of particles per unit volume, and x is the distance measured parallel to the direction of flow; D is the diffusion coefficient, which is a property of the particular dopant and is an exponential function of temperature.
Fast diffusing dopant species, i.e., dopants having a high diffusion coefficient, such as phosphorous (n-type) and boron (p-type), are difficult to control within the crystalline silicon lattice. Thus, when these dopants are used, shallow diffusion region junctions and the lateral containment necessary for isolation of diffused dopant regions are difficult to achieve.
Both Meyers et al, "Ge.sup.+ preamorphization of silicon: effects of dose and very low temperature thermal treatments on extended defect formation during subsequent SPE", Proc. Mat. Res. Soc., 52, 107 (1986), and Sadana et al, "Germanium implantation into silicon: an alternative preamorphization rapid thermal annealing procedure for shallow junction formation", Proc. Mat. Res. Soc., 23, 303 (1984), have reported the use of germanium for preamorphizing silicon to reduce dopant diffusion. According to this technique, germanium atoms are introduced into the silicon lattice to destroy crystallinity in the area of introduction. Then, the active dopant species is introduced into the preamorphized area. The structure is then subjected to a high temperature annealing procedure which results in the creation of the desired dopant regions in a reconstructed crystalline lattice having germanium atoms at substitutional sites. The purpose of the preamorphitation is to minimize dopant channeling in the creation of the dopant regions.
Germanium is chosen as the preamorphizing agent because of its unlimited solid-solubility in silicon. Furthermore, as a member of the same group as silicon, germanium does not change the electronic configuration of the silicon lattice. It also has been found that the damage sites in the silicon lattice introduced by germanium act as gettering centers.
While the germanium preamorphization procedure reduces the damage caused to the silicon lattice in the formation of diffused dopant regions, it has little effect on the dopant diffusion control problems mentioned above.